Grease and line a 20cm (8 inch) square cake tin. Break the Chocolate into pieces and place in a saucepan. Add the butter and put over a very low heat, stirring occasionally, until melted. Remove from the heat and transfer to a heatproof bowl and leave to cool for 10 minutes.
Stir in the digestives, marshmallows and you favourite chocolate confectionary. Mix well to make sure all the pieces are covered in the chocolate mixture and spoon into the prepared tin and press the top down gently. Cool and then chill for at least an hour until set.
To serve, stand at room temperature for about 15 minutes, then remove from the tin and peel away the lining paper. Cut into 20 pieces using a warm knife and serve.